The strengthening of glass articles through the introduction of compressive stresses into the surfaces of the articles can be accomplished by thermal tempering, ion-exchange or chemical tempering, and the lamination of glass surface layers to glass core layers. Thermal tempering involves rapidly cooling the surfaces of a hot glass article to set the surface glass while allowing the core glass to contract through slower cooling. Ion-exchange strengthening or so-called chemical tempering typically involves exchanging large mobile ions from the surfaces of the glass with smaller ions in the interior of glass which can introduce compressive stresses into the glass surfaces. In laminar strengthening, glass surface layers or skins of relatively low thermal expansion are fused to glass core layers of relatively high thermal expansion so that compressive stress can develop in skins as the laminated articles are cooled following fusion.
While each of these known methods of glass strengthening has been employed successfully to improve the suitability of glass for a number of existing technical applications, new applications have imposed higher demands on the ability of glass materials to resist surface damage in use. In some flat glass applications, for example, the damage resistance of one of the surfaces of the glass needs to be superior to the damage resistance of the other surface, because one of the surfaces sees more abuse in day-to-day use than the other surface. Touch screen displays are an example of an application where increased surface damage resistance in the active or exposed surface of the displays is presently required.
In most instances the known methods of glass strengthening have been used to induce uniform compressive stresses and uniform depths of surface compression on each of the two surfaces of the glass article being strengthened. While resistance to surface damage can be improved in some cases through modifications of these methods to increase the levels of surface compression in the articles, the results are not satisfactory for a number of applications. One disadvantage, for example, is that increasing surface compression can correspondingly increase core tension. High core tensions in strengthened glass articles can undesirably increase the fracture energy of the article in the event of breakage.